The Vulkan backend uses a vkImage that matches the number
of effective levels in the GL texture. This is due to the fact
that GL textures can have really strange layouts that only make
sense when base level and max level are applied.

For instance, take the following layout with disjoint mip levels:

  Level 0: 4x4 RGBA
  Level 1: 2x2 RGBA
  Level 2: 10x10 RGB

If base level is set to zero and max level is set to 1, the image is
still considered mip-complete:

  Level 0: 4x4 RGBA  ==> Base Level 0 ==>  Level 0: 4x4 RGBA
  Level 1: 2x2 RGBA  ==> Max Level 1  ==>  Level 1: 2x2 RGBA
  Level 2: 10x10 RGB

If base and max level are then both set to 2, the texture is still
considered complete, but of a different size and format:

  Level 0: 4x4 RGBA
  Level 1: 2x2 RGBA
  Level 2: 10x10 RGB ==> Base/Max Level 2 ==> Level 2: 10x10 RGB

When the base or max level is changed, we must recreate the vkImage to
match the new level count.

To support that, we:

 - Stage updates from the current image to the new image
 - Only stage updates if there aren't already staged updates for a level
 - Free the current image and so it can be recreated at the next draw

This CL does the following:

 - Refactors TextureVk::copyImageDataToBuffer to support staging updates
   without flush
 - Adds TextureVk::copyImageDataToBufferAndGetData to support previous
   use model
 - Adds TextureVk::changeLevels, triggered during syncState, which stages
   updates and releases the current image.
 - Updates ImageHelper::flushStagedUpdates to understand base/max levels
 - Updates TextureVk::ensureImageInitialized and TextureVk::generateMipmap
   to account for base/max level
 - Tracks base and max levels in ImageHelper
 - Adds ImageHelper::stageSubresourceUpdateFromBuffer to support
   this use case
 - Adds ImageHelper::isUpdateStaged to determine if changeLevels
   should propagate data
 - Makes gl::TextureTypeToTarget available for use outside of ImageIndex
 - Enables several deqp and end2end tests

Bug: angleproject:3148
Test: dEQP-GLES3.functional.texture.mipmap.*base_level*
Test: dEQP-GLES3.functional.texture.mipmap.*max_level*
Change-Id: I14ca071c9c62eb310dfed7ef9290dc65fc3ff696
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1776933Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Commit-Queue: Cody Northrop <cnorthrop@google.com>